1. Field of the Invention
The present invention relates to a satellite radiowave receiver which receives radiowave from positioning satellites to obtain satellite signals and a satellite radiowave receiving method thereof.
2. Description of Related Art
Conventionally, portable devices such as clocks/watches and GPS loggers which obtain time information and position information by receiving radiowaves from positioning satellites according to GNSS (Global Navigation Satellite System) such as GPS (Global Positioning System) are known. Due to their power restriction, low power consumption components are used and power saving operation is carried out in such portable devices.
For example, JP 2008-32637 discloses a technique to cut off the receiving operation during a period other than the timing when time information is transmitted among satellite signals when obtaining time information from positioning satellites. Further, JP 2008-51529 discloses a time correction device which is provided with an advanced sensor to obtain position information by receiving data from four satellites and determine the time zone only when it is determined that moving by an airplane took place.
Transmission frequencies of radiowaves which are transmitted from positioning satellites are the same among a plurality of positioning satellites. For example, one of the civilian radiowave frequency transmitted from positioning satellites of GPS (GPS satellites) is L1-band of 1.57542 GHz. However, because the doppler effect occurs due to relative movement between the GPS satellites and the portable device because each of the GPS satellites moves in its orbit at high speed, and the receiving frequency at the portable device be a frequency deviated from each of the transmission frequencies. Therefore, when receiving radiowave from positioning satellites by a porable device which does not have its own current position data and orbit information of the positioning satellites in advance, receiving frequencies for receiving radiowaves from the positioning satellites need to be searched within a range which may change due to the doppler effect.
On the other hand, with regard to an oscillation circuit which demodulates satellite signals from the received radiowaves and generates frequency signals for carrying out digital sampling in the portable device, each of the oscillation frequencies thereof includes error. Further, this error in each oscillation frequency is even greater in a small low power consumption oscillation circuit which can be mounted in the portable device. Therefore, when searching for radiowave transmitted from positioning satellites, the search needs to be carried out in a broad range of receiving frequencies including the range of errors in the oscillation frequencies. Thus, the searching time be lengthy and the amount of power consumption increases.